1. Field of the Invention
The present invention relates to an optical absolute position encoder for measuring the position of a machine tool such as a turning machine and a milling machine or an apparatus for manufacturing semiconductors.
2. Description of the Related Art
FIG. 1 is a perspective structural view which illustrates an optical system of a conventional optical absolute position encoder. Referring to FIG. 1, the optical absolute-position encoder comprises a collimator lens 12 for forming measuring light beams La, which have been emitted from a light emitting element 11 such as an LED (Light Emitting Diode) or a lamp, into parallel light beams Lb. The optical absolute position encoder further comprises a first scale 13 in which n (n is an integer) grating tracks t.sub.1, t.sub.2, . . . , and t.sub.n are formed in parallel to one another on the surface thereof, the grating tracks t.sub.1, t.sub.2, . . . , and t.sub.n being arranged in such a manner that portions (hereinafter called "transmissible portions") 13A for transmitting the parallel light beams Lb, which have passed through the collimator lens 12 and portions (hereinafter called "non-transmissible portions") 13B for blocking the same are repeated at predetermined lengths (hereinafter called a "grating pitches"). The optical absolute position encoder further comprises a second scale 14 in which transmissible windows 14A.sub.1, 14A.sub. 2, . . . , and 14A.sub.n for transmitting light beams (omitted from illustration), which have passed through the above-described transmissible portions 13A, are formed to correspond to the grating tracks t.sub.1, t.sub.2, . . . , and t.sub.n of the first scale 13. In addition, the optical absolute position encoder comprises photoelectrical converting elements 15-1, 15-2, . . . , and 15-n disposed to confront the transmissible windows 14A.sub.1, 14A.sub.2, . . . , and 14A.sub.n of the second scale 14 for respectively converting light beams Lc.sub.1, Lc.sub.2, . . . , and Lc.sub.n, which have passed through the corresponding transmissible windows 14A.sub.1, 14A.sub.2, . . . , and 14A.sub.n, into electrical signals which correspond to the intensities of the light beams Lc.sub.1, Lc.sub.2, . . . , and Lc.sub.n.
The first scale 13 for use in the optical system 10 of the optical absolute position encoder thus-constituted is provided with an alternative binary code (a gray code) in which the grating pitches P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.n-1, and P.sub.n of the adjacent grating tracks t.sub.1 and t.sub.2, t.sub.2 and t.sub.3, . . . , t.sub.n-1 and t.sub.n as shown in FIG. 2 hold a proportional relationship of 1:2. The light beams Lc.sub.1, Lc.sub.2, Lc.sub.3, . . . , Lc.sub.n-1 and Lc.sub.n pass through the transmissible portions 13A of each of the grating tracks t.sub.1, t.sub.2, t.sub.3, . . . , t.sub.n-1 and t.sub.n of the first scale 13 and the transmissible windows 14A.sub.1, 14A.sub.2, 14A.sub.3, . . . , 14A.sub.n-1 and 14A.sub.n of the second scale 14 which correspond to the grating tracks t.sub.1, t.sub.2, t.sub.3, . . . , t.sub.n-1 and t.sub.n. Then, the light beams Lc.sub.1, Lc.sub.2, Lc.sub.3, . . . , Lc.sub.n-1 and Lc.sub.n are made incident upon the photoelectrical converting elements 15-1, 15-2, 15-3, . . . , 15-n-1 and 15-n. As a result, the intensities of the above-described light beams Lc.sub.1, Lc.sub.2, Lc.sub.3, . . . , Lc.sub.n-1 and Lc.sub.n are respectively periodically changed in accordance with the lengthwise directional movement (designated by an arrow x) of the first scale 13. As a result, the electric signals, which are generated by the converting operations performed in the photoelectrical converting elements 15-1, 15-2, 15-3, . . . , 15-n-1 and 15-n, are also changed in accordance with the above-described changes of the intensities. In FIG. 3, the abscissa axis stands for the lengthwise directional displacement ml of the first scale 13 and the ordinate axis stands for electric signals S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1 and S.sub.n generated by the conversion performed by the corresponding photoelectrical converting elements 15-1, 15-2, 15-3, . . . , 15-n-1 and 15-n. As can be clearly seen from FIG. 3, each of the electrical signals S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1 and S.sub.n are periodically changed. The electrical signals S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1 and S.sub.n are then, as shown in a block diagram shown in FIG. 4 which illustrates the optical absolute position encoder, digitized into signals d.sub.1, d.sub.2, d.sub.3, . . . , d.sub.n-1 and d.sub.n by a comparator 20. Then, they are converted by a decoder 30, from the alternative binary code into absolute position data D in a desired form such as a pure binary code, a BCD code or the like.
An optical absolute position encoder for measuring a position must have a further improved position detecting resolution so as to detect a further small displacement quantity. Furthermore, there is a desire of an optical absolute position encoder capable of detecting the absolute position for a longer stroke. However, optical absolute position encoders of the type described above have an unsatisfactory minimum position detecting resolution which is substantially the same as the grating pitch P.sub.n of the grating track t.sub.n which has been divided into minimum sections. Furthermore, the absolute position detecting stroke is substantially the same as the grating pitch P.sub.1 of the grating track t.sub.1 which has been divided into maximum sections. Therefore, the position detecting resolution cannot be improved and the absolute position detecting stroke cannot be lengthened while reducing the overall size of the optical absolute position encoder because the number n of the grating tracks is increased excessively. Another problem will take place in that the number of the photoelectrical converting elements, comparators or the like which are the elements of the optical absolute position encoder is increased undesirably.